The ultimate objective of the proposed research is to develop a photochemical technique for the localized delivery of pharmaceutical agents in the mammalian body. Liposomes (microscopic bilayer phospholipid vesicles) have been used successfully by a number of workers to entrap a variety of drugs for intravenous injection. The approach in the present work is to incorporate photodynamic sensitizers into the drug-containing liposomes; illumination increases the permeability of the sensitized liposomes so that the drug leaks out more rapidly. Thus injected photosensitive liposomes should deliver their contents more efficiently in localized areas of an organism exposed to high intensity light; the technique would be restricted to those parts of the body which can be illuminated. The research program consists primarily of a detailed study of the development of techniques for the preparation of highly photosensitive lipsomes. In particular, the efficiencies and mechanisms of photosensitization of different kinds of sensitizing dyes, the effects of dye location in the liposomal structure and the molecular photochemistry of sensitized reactions in liposomes leading to increased permeability are being examined using liposomes of different sizes and chemical composition.